1. Field of the Invention
This invention relates to a locomotion control system for a legged mobile robot and more particularly to a system in which an inclinatory angle of a floor on which the robot actually walks is estimated and an error or deviation between the floor and a floor supposed in a desired walking pattern (gait) such that the robot walks thereon, is calculated, and the posture of the robot is corrected such that floor reaction force acting to the robot does not shift, which would otherwise occur due to the error, from that desired. And this invention further relates to a system for correcting an output of an inclinometer mounted on the legged robot also utilizing the error thus determined.
2. Description of the Prior Art
Legged mobile robots, in particular legged mobile robots using biped locomotion, are taught by Japanese Laid-open Patent Publication Nos. 62(1987)-97005 and 63(1988)-150176. A general discussion of robots including this type of robots can be found in Robotics Handbook, Robotic Society of Japan, published on Oct. 20, 1990.
The stability of legged mobile robots, in particular legged mobile robots using biped locomotion, is intrinsically low. Because of this, the assignee proposed earlier a technique in which a mechanical compliance mechanism was provided at the robot's individual feet and moment of force acting to the robot was detected by a sensor and was controlled to a predetermined value, in Japanese Patent Application No. 4(1992)-137881 filed on Apr. 30, 1993 which was also filed in the United States on Apr. 19, 1993 under the number of 08/049494. Further, the assignee proposed another technique in which the ZMP (zero moment point) was detected and when it was found to be out of a desired position, the robot feet were driven on a floor in opposite directions such that the error was decreased, in Japanese Patent Application No. 4(1992)-137884 filed on Apr. 30, 1992 which was also filed in the United States on Apr. 30, 1993 under the Ser. No. of 08/056067. In the technique, the robot was thus approximated as an inverted pendulum and was controlled to walk stably on a floor. Furthermore, the assignee proposed a technique in which a feedback correction was applied to the control just mentioned above in response to the robot body's posture inclination (inclinatory angle) so as to enhance the posture stability, in Japanese Patent Application No. 4(1992)-137885 filed on Apr. 30, 1990, whose content was included in the U.S. application Ser. No. 08/056067 referred to in the above.
In the control of a legged mobile robot, a desired walking pattern (gait) is preestablished in terms of positions/orientations of the feet and hip joints etc. such that it satisfies dynamic equilibrium. In the techniques earlier proposed, posture inclination of the robot converges on a desired value expected in the desired walking pattern provided that the configuration such as inclinatory angle of a floor supposed in the desired walking pattern coincides with that of a floor on which the robot is actually walking and at the time of walking no disturbance exists. The floor supposed in the desired walking pattern is hereinafter referred to as "supposed floor". More specifically, when calling a deviation or error between the actual and desired posture inclination as "posture inclination control error", the error converges to zero. However, if the actual floor is not the same as the floor supposed in configuration, namely if there is a configuration error therebetween, the error becomes a disturbance, resulting the posture inclination control error to happen. For example, when there is an inclination error between the actual and supposed floors, if the posture inclination feedback control is conducted using PD control laws, there remains a steady-state error in proportion to the inclination error.
This will be explained with reference to FIGS. 26 and 27. As illustrated in FIG. 26, let is be presumed that the actual floor AF inclines by an angle .theta. floor (.THETA.2) while the supposed floor SF is level and hence the robot body's inclination shifts in the direction of arrow A1 from a desired posture inclination by an angle .theta.4. The robot's ankle joint is then driven in the direction shown in the figure by an amount .theta..multidot.K made (.THETA.6) up of the detected .theta.4 multiplied by a proportional gain K such that the ankle is bent by the amount as illustrated in FIG. 27. As a result, although the robot posture is restored in the direction of arrow A2 to stable state, there still remains a steady-state error (.theta.8) offset, as shown. Namely, from geometric relationship between the robot and floor, EQU Ankle joint's bending angle=.theta.floor-.theta.offset
And in the steady-state, from the feedback control laws EQU Ankle joint's bending angle=K.multidot..theta.offset
Hence, EQU .theta.offset=.theta.floor/(K+1)
Here, if the proportional gain K is set to be infinite, the steady-state error will almost be zero. Since, however, the proportional gain K is a finite value, the error still remains as a finite value. The steady-state error will nevertheless be decreased to zero by the following methods. 1. To use integral type control law such as PID or I-PD in the posture inclination control. 2. To configure the control as torque control in which external force acting to the robot legs is detected and is fed back to ankle joint displacement command using integral type control law such as PID or I-PD.
The methods are however disadvantageous in that the integral element, if used, results a delay in the open-loop transfer function, making the system unstable. That is, a phase lag will occur and the system will be liable to vibrate.
The same purpose can be achieved by using more conventional methods listed as follows.
3. Cutting ankle joint displacement's feedback and to configure the ankle joint control as torque control.
4. To configure the ankle joint control as joint motor's current control
Since methods 3 and 4 do not utilize displacement control, they are free from floor configuration variations such as inclination, difference in level or bumps. However, if method 3 or 4 is used, it has to be switched to displacement control during robot's free leg phase and what is worse, an impact may often occur at the transition.
The object of the invention is therefore to eliminate the aforesaid shortcomings of the prior art and to provide a locomotion control system for a legged mobile robot in which the aforesaid floor configuration error is estimated and robot posture is corrected such that floor reaction force does not shift, which would otherwise occur due to the configuration error, from that desired.
Another object of the invention is to provide a locomotion control system for a legged mobile robot in which the floor configuration error is estimated and in response thereto, the robot is controlled in an appropriate manner such as to cease its locomotion.
Further object of the invention is to provide a locomotion control system for a legged mobile robot in which a steady-state error in the robot's posture inclination control is substantially decreased, without degrading the stability of the control, to zero.
Aside from the above, a legged mobile robot is usually equipped with an inclinometer for detecting the posture inclination of the robot. The inclinometer is categorized to three types. First typed one has a detector for detecting inclinatory angular velocity and estimates inclinatory angle by integrating the detector's outputs. Second typed one detects inclinatory angle using a detector for gravitational direction and the third typed one is the combination of the first and second typed inclinometers. Among them, the first type of the inclinometer is disadvantageous in that estimation error is liable to grow if the detector's output drifts. The second typed one is advantageous in that estimation error does not increase, but is disadvantageous in that detection error may vary due to the occurrence of temperature drift in the gravitational direction's detector or influence of inertial force resulting from the robot's acceleration/deceleration motion. The last type of the inclinometer aims to cancel the drawbacks of the first and second typed ones and is free from the error's increase and the influence from inertial force is considerably decreased. However, the influence of the temperature drift is never eliminated.
Still further object of the invention is therefore to eliminate the drawbacks of the prior art and to provide a system for correcting an output of an inclinometer mounted on the legged mobile robot in which the robot's posture relative to a known floor configuration is estimated to determine the robot's posture inclination and based thereon, the inclinometer's output is corrected with accuracy.